Communication satellite payloads typically use high power amplifiers to increase power of received signals. The high power amplifiers are grouped together in redundancy rings, such that each high power amplifier within the ring has the same operating frequency range, bandwidth, and output power. For example, a commercial satellite may have forty 32 Ku-Band 120 W traveling wave tube amplifiers, 24 C-Band 40 W traveling wave tube amplifiers, and 38 90 W Ka-Band traveling wave tube amplifiers. Each high power amplifier is required to meet down link power or effective isotropic radiated power (EIRP) requirements equivalent to adjacent high power amplifiers contained within a redundancy ring. However, different paths through the redundancy ring have varying amounts of loss and not all of the high power amplifiers have identical EIRP requirements. Therefore, extra power is provided to high power amplifiers that have lower EIRP requirements. This is not power efficient.
Additionally, current high power amplifiers are limited in that they are designed and manufactured for one specific performance application. A performance application may be associated with providing service to a specific service area or serving or providing backup to a particular business service plan. Differing performance applications require different output RF power levels.
In conventional satellite payloads a high power amplifier typically includes a power supply that monitors cathode current of a high power amplifier and adjusts anode voltage to maintain a constant cathode current via an analog feedback loop. Over time, as a cathode of the high power amplifier degrades, the power supply compensates for this change by adjusting anode voltage to maintain a constant cathode current for a single designed performance application.
U.S. patent application entitled “On Orbit Variable Power High Power Amplifiers for a Satellite Communications System” provides a power system for a satellite that determines minimum EIRP for a high power amplifier and adjusts saturated power output of the high power amplifier to the minimum EIRP, thereby reducing power consumption of the system. Reducing the amount of power consumed by various components is desired since it not only conserves energy but also allows additional transponders to be placed upon a satellite to generate additional revenue.
Unfortunately, although, the above stated application provides a high power amplifier with variable output power that may be externally adjusted when in orbit it does not provide a technique for maintaining constant overall gain of the amplifier, thus also limiting its use for multiple performance applications. For example, when RF power of a high power amplifier, such as a flexible traveling wave tube as used in the stated application, is increased by a factor of two (3 dB), gain of the amplifier can increase by a factor of thirty-two (15 dB). Magnitude difference of this gain change is too large to handle in an analog environment and as such limits and potentially prevents the high power amplifier from being used in multiple performance applications.
It would therefore be desirable to provide a high power amplifier system that has reduced operating power consumption, that has an externally controllable variable RF power output, and that is capable of being used for multiple performance applications.